21904 ÷ 7 = 3129 R1
We're going to perform division on 21904 with 7 as the divisor.

Step 1:
7 goes into 2 0 times with a remainder of 2.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 2 to get 2.
Bring next digit (1) of the dividend behind the 2 and repeat the process: 21 / 7

Step 2:
7 goes into 21 3 times with a remainder of 0.
Write down 3 as next digit of of the result. 
Result so far: 3
Subtract 21 from 21 to get 0.
Bring next digit (9) of the dividend behind the 0 and repeat the process: 9 / 7

Step 3:
7 goes into 9 1 times with a remainder of 2.
Write down 1 as next digit of of the result. 
Result so far: 31
Subtract 7 from 9 to get 2.
Bring next digit (0) of the dividend behind the 2 and repeat the process: 20 / 7

Step 4:
7 goes into 20 2 times with a remainder of 6.
Write down 2 as next digit of of the result. 
Result so far: 312
Subtract 14 from 20 to get 6.
Bring next digit (4) of the dividend behind the 6 and repeat the process: 64 / 7

Step 5:
7 goes into 64 9 times with a remainder of 1.
Write down 9 as next digit of of the result. 
Result so far: 3129
Subtract 63 from 64 to get 1.

The final result is 3129 with a remainder of 1.